1. Field of the Invention
The present invention relates to an electric motor having an air tightness test hole which communicates with an internal space of the electric motor, in a housing of the electric motor.
2. Description of the Related Art
An electric motor which is used for an industrial machine including a machine tool or an industrial robot is required to have high oil resistance and drip proofness, and the electric motor needs to have high air tightness. A method of checking or testing air tightness of an electric motor in electric motor manufacturing process is a method of checking leakage of air to an outside of an electric motor or an inflow of air to an inside from an outside of the electric motor by applying a positive pressure or a negative pressure to an interior of the electric motor.
As illustrated in FIG. 9, a conventional electric motor 10 is formed with a stator 14 which has a winding (not illustrated), housings 12 and 16 which are attached to both ends of this stator 14 and a rotor (not illustrated) which has an output shaft 20. The rotor and the winding of the stator 14 are arranged in space formed with the stator 14 and the housings 12 and 16 at both ends of the stator 14.
Ball bearings 22 are attached to center positions of the housings 12 and 16, and the output shaft 20 provided to the rotor is supported by the ball bearings 22. Further, the stator 14 has a power terminal 18 which is a terminal which receives an input of power for driving the electric motor 10, and the housing 16 has a position/speed detector 24 which detects a rotation position and a speed of the output shaft 20.
Regarding such an electric motor, when an air tightness test of an interior of the electric motor is conducted, the air tightness test can be conducted without detaching the ball bearings 22 in a case where sealing performance of the ball bearings 22 is not high. However, in a case where the ball bearings 22 having seals of increased air tightness are used in order to prevent bearing grease in particular from flowing out, an air tightness test is conducted with the ball bearings 22 detached from the electric motor.
As another technique, Japanese Patent Application Laid-Open No. 2008-11591 discloses a technique of covering entirety of a decelerating device and an electric motor by means of an airtight container formed with a first shell, a second shell and a third shell, providing an air tightness test hole to this air tight container and testing air tightness of the entire structure formed with the decelerating device and the electric motor. This technique is a technique of testing air tightness of the entire structure formed with the decelerating device and the electric motor, and cannot test air tightness of the electric motor itself.
Further, when bearings which have oil seals of high sealing performance are used as bearings which support the output shaft 20 of the rotor to prevent bearing grease in particular from flowing out, air cannot pass through these bearings even when the oil seals are detached, and therefore it is not possible to apply the pressure to the interior of the electric motor. Hence, unless a portion which communicates with the inside of the electric motor is additionally provided by detaching the bearings or pushing out the oil seals of the bearings, it is not possible to apply the pressure to the interior of the electric motor. That is, when bearings of high sealing performance as described above are used in an electric motor, man-hours of an air tightness test of this electric motor increases.